Analogue Pitch Shifter

Description: This is two three pole bandpass filters with the HPF Fo about 50Hz and the LPF Fo about 15kHz. This limits the input bandwidth to the phase sequence array, which can produce 90° phase shifts only over a limited bandwidth. The array is driven with a true and inverted signal, and the array outputs are buffered to feed the following circuitry. The phase sequence array itself has a gradual -4dB dip mid-band and the input filters partly compensate for this. The exact values of the capacitors are not critical but each value set should be matched to 1%. The S_0 and S_90 output buffers provide convenient input points for the carrier feedthrough trims. +/-5V 100mA regulators provide local voltage references. The two lower transconductance amplifiers form a standard double integrator bandpass-lowpass filter as shown in the Nat Semi data sheet for the LM13700. These lower TAs are active for positive control voltages, and the upper TAs are effectively turned off by being fed no bias current; These play no part when the CV is positive. The TL074 op-amps are merely buffers to enable easier choice of resistor values on the TA inputs and to make sure that the integrator capacitors are not loaded excessively. The resistor values set Q to be >1. Feedback from the bandpass output to the non-inverting input via the diode limiter provides feedback to make the filter oscillate. Adjusting the trimmer gives output amplitude trimming and allows the drive to the diodes to be set to a level that ensures oscillation but minimizes distortion. This fairly standard configuration gives quadrature outputs from the two buffers with the highest distortion product about 40dB down on the fundamental. For negative control voltages the upper TAs are fed bias current and the lower TAs are shut off. The upper TAs work in exactly the same way as the lower ones but they are cross connected to the inputs and integrator caps. In this way the in-phase and quadrature outputs are reversed for negative CV, thus creating an inherently smooth transition to what can be considered to be a negative frequency. In operation, viewing the outputs on an X-Y trace the circular rotation of the dot becomes slower as the CV is adjusted to near zero and then reverses direction perfectly as the CV goes negative, without any unwanted crossing of the circle or drifting off beyond the circle. This is true provided that the transition to -ve CV does not take many seconds. The two current sources are fairly self-explanatory, with the upper source operating for -ve CV and vice versa. The gain trimmer on the upper source allows the oscillator to be adjusted such that a given -ve CV gives the same frequency as the equal magnitude +ve CV. This compensates for differences in Gm of the two separate dual TA packages. The diodes across the op-amps avoid heavy output saturation when that source is turned off. Low Vf Schottky diodes are better in this application. Here we have the modulators fed with the audio and carrier signals. The output of the oscillator is so-called carrier because this circuit is identical to that used to create RF sideband signals, only the RF carrier frequency is actually in the audio band in this case. The carrier signal is potted down from 2Vpp to about 20mVpp which is a suitable level for driving these modulators in their linear mode. This circuit can be thought of either as a sideband generator running at AF as described or as a continually sine-weight panning between the 0, 90, 180, 270, 0 outputs of the phase sequence array. For a given sine input to the array, this scanning creates a situation analogous to doppler shift where the listener is running towards or away from a sound source and therefore intercepting more or fewer wavefronts. Readers may note the similarity here between this pitch shifter and the original Hammond organ scanning vibrato, though the Hammond LC line is closer to a true delay than this, wholly phase shift oriented design. For this reason, the Hammond vibrato preserves the harmonic relations of the signal better than this design, though far from perfectly. The phase-shift only nature of the pitch shifter allows us to "scan" and thus frequency shift, in one direction continuously, while the Hammond LC line has to be scanned up and down due to the time discontinuity that would be experienced when trying to flyback to the start of an LC line. The output from the second board is taken back to two summing amplifiers on this board. One sums the output of the two boards to form a duo chorus output. The other sums the output of both boards plus the input to board 1 to form a trio chorus output. The signal switching of the front panel jacks is arranged so that the input to board 1 is fed to both board 1 and board 2 when there is no jack connected to the board 2 input. Both oscillation modes need to be trimmed to give equal output amplitude using the diode limiter drive trimmers. A front panel switch selects LOG or LIN CV and a range switch reduces the oscillator frequency when closed. For +/-10V linear CV the oscillator should give about +/-10kHz.

The circuit consists of two three-pole bandpass filters, specifically designed to filter signals within the specified frequency range of 50 Hz to 15 kHz. The high-pass filter (HPF) is tuned to a cutoff frequency (Fo) of approximately 50 Hz, while the low-pass filter (LPF) is set to a cutoff frequency of about 15 kHz. This configuration is essential for limiting the input bandwidth to the phase sequence array, which generates 90-degree phase shifts within a constrained frequency range. The phase sequence array is driven by both true and inverted signals, and its outputs are buffered for further processing.

The phase sequence array exhibits a -4 dB dip in the mid-band frequency response, and the input filters are designed to partially compensate for this dip. Precision in the capacitor values is not critical; however, it is recommended that each capacitor value be matched to within 1% tolerance to ensure consistent performance. The outputs, S_0 and S_90, from the phase sequence array are buffered for easy integration with the subsequent circuitry, and local voltage references are provided by +/-5V, 100mA regulators.

The two lower transconductance amplifiers (TAs) create a standard double integrator bandpass-lowpass filter configuration, as detailed in the National Semiconductor data sheet for the LM13700. These amplifiers operate under positive control voltages, while the upper TAs remain inactive when no bias current is applied. The TL074 operational amplifiers serve as buffers to facilitate the selection of resistor values on the TA inputs and to prevent excessive loading on the integrator capacitors. The resistor values are chosen to ensure that the quality factor (Q) remains greater than 1.

Feedback from the bandpass output to the non-inverting input through a diode limiter induces oscillation within the filter. The amplitude of the output can be adjusted using a trimmer, which also allows for optimization of the diode drive level to maintain oscillation with minimal distortion. This configuration yields quadrature outputs from the two buffers, with the highest distortion product being approximately 40 dB below the fundamental frequency.

In scenarios where negative control voltages are applied, the upper TAs receive bias current while the lower TAs are disabled. The upper TAs mirror the functionality of the lower ones, but their inputs and integrator capacitors are cross-connected. This design allows for a seamless transition to negative frequencies, resulting in a smooth change in output characteristics. When observing the outputs on an X-Y display, the rotation of the dot slows as the control voltage approaches zero and reverses direction as the control voltage transitions to negative, without any erratic behavior.

The circuit also incorporates two current sources, with the upper source activated for negative control voltages and vice versa. A gain trimmer on the upper current source enables frequency matching between equal magnitude positive and negative control voltages, compensating for any discrepancies in transconductance (Gm) between the dual TA packages. Schottky diodes are utilized across the op-amps to prevent saturation during operation.

The modulators are supplied with audio and carrier signals, with the oscillator output functioning as a carrier signal. This oscillator design is analogous to those used for RF sideband signal generation, albeit operating within the audio frequency range. The carrier signal is attenuated from 2 Vpp to approximately 20 mVpp, suitable for driving the modulators in their linear operating range. This setup can be conceptualized either as a sideband generator operating at audio frequencies or as a continuous sine-weighted panning across the outputs of the phase sequence array.

The system exhibits characteristics reminiscent of the Hammond organ's scanning vibrato, although the design is primarily focused on phase shifting rather than true delay, which allows for continuous frequency shifting in one direction. The output from the second board is routed to two summing amplifiers, one of which combines the outputs from both boards to create a duo chorus effect, while the other sums the outputs from both boards along with the input to the first board for a trio chorus output. Front panel jacks are configured such that the input to the first board is routed to both boards when no connection is made to the second board's input.

Both oscillation modes require careful trimming to ensure equal output amplitudes, achieved through the diode limiter drive trimmers. A front panel switch allows selection between logarithmic and linear control voltage responses, and a range switch is provided to reduce the oscillator frequency when activated. Under a +/-10V linear control voltage, the oscillator is expected to produce frequencies in the range of approximately +/-10 kHz.This is two three pole bandpass filters with the HPF Fo about 50Hz and the LPF Fo about 15kHz. This limits the input bandwidth to the phase sequence array, which can produce 90° phase shifts only over a limited bandwidth. The array is driven with a true and inverted signal, and the array outputs are buffered to feed the following circuitry.

The phase sequence array itself has a gradual -4dB dip mid-band and the input filters partly compensate for this. The exact values of the capacitors are not critical but each value set should be matched to 1%.The S_0 and S_90 output buffers provide convenient input points for the carrier feedthrough trims.

+/-5V 100mA regulators provide local voltage references. The two lower transconductance amplifiers form a standard double integrator bandpass-lowpass filter as shown in the Nat Semi data sheet for the LM13700. These lower TAs are active for positive control voltages, and the upper TAs are effectively turned off by being fed no bias current; These play no part when the CV is positive.

The TL074 op-amps are merely buffers to enable easier choice of resistor values on the TA inputs and to make sure that the integrator capacitors are not loaded excessively. The resistor values set Q to be >1. Feedback from the bandpass output to the non-inverting input via the diode limiter provides feedback to make the filter oscillate.

Adjusting the trimmer gives output amplitude trimming and allows the drive to the diodes to be set to a level that ensures oscillation but minimises distortion. This fairly standard configuration gives quadrature outputs from the two buffers with the highest distortion product about 40dB down on the fundamental.

For negative control voltages the upper TAs are fed bias current and the lower TAs are shut off. The upper TAs work in exactly the same way as the lower ones but they are cross connected to the inputs and integrator caps. In this way the in-phase and quadrature outputs are reversed for negative CV, thus creating an inherently smooth transition to what can be considered to be a negative frequency.

In operation, viewing the outputs on an X-Y trace the circular rotation of the dot becomes slower as the CV is adjusted to near zero and then reverses direction perfectly as the CV goes negative, without any unwanted crossing of the circle or drifting off beyond the circle. This is true provided that the transition to -ve CV does not take many seconds. The two current sources are fairly self explanatory, with the upper source operating for -ve CV and vice versa.

The gain trimmer on the upper source allows the oscillator to be adjusted such that a given -ve CV gives the same frequency as the equal magnitude +ve CV. This compensates for differences in Gm of the two separate dual TA packages. The diodes across the op-amps avoid heavy output saturation when that source is turned off. Low Vf Schottky diodes are better in this application. Here we have the modulators fed with the audio and carrier signals. The output of the oscillator is so-called carrier because this circuit is identical to that used to create RF sideband signals, only the RF carrier frequency is actually in the audio band in this case.

The carrier signal is potted down from 2Vpp to about 20mVpp which is a suitable level for driving these modulators in their linear mode. This circuit can be thought of either as a sideband generator running at AF as described or as a continually sine-weight panning between the 0, 90,180 270, 0 .

outputs of the phase sequence array. For a given sine input to the array, this scanning creates a situation analogous to doppler shift where the listener is running towards or away from a sound source and therefore intercepting more or fewer wavefronts. Readers may note the similarity here between this pitch shifter and the original Hammond organ scanning vibrato, though the Hammond LC line is closer to a true delay than this, wholly phase shift oriented design.

For this reason the Hammond vibrato preserves the harmonic relations of the signal better than this design, though far from perfectly. The phase-shift only nature of the pitch shifter allows us to "scan" and thus frequency shift, in one direction continously, while the Hammond LC line has to be scanned up and down due to the time discontinuity that would be experienced when trying to flyback to the start of an LC line.

The output from the second board is taken back to two summing amplifiers on this board. One sums the output of the two boards to form a duo chorus output. The other sums the output of both boards plus the input to board 1 to form a trio chorus output. The signal switching of the front panel jacks is arranged so that the input to board 1 is fed to both board 1 and board 2 when there is no jack connected to the board 2 input. Both oscillation modes need to be trimmed to give equal output amplitude using the diode limiter drive trimmers.

A front panel switch selects LOG or LIN CV and a range switch reduces the oscillator frequency when closed. For +/-10V linear CV the oscillator should give about +/-10kHz.